1. Field of the Invention
The present invention relates to outboard motors for watercraft, and in particular, to an exhaust system for an outboard motor.
2. Description of the Related Art
Outboard motors containing internal combustion engines are commonly used for powering watercraft. A housing, which is mounted to a transom of the watercraft, typically encloses the engine. Rotation of a crankshaft of the internal combustion engine drives a driveshaft. The driveshaft drives a water propulsion device, such as a propeller. When the watercraft operates, the propeller is submerged beneath a water surface. Rotation of the propeller moves the watercraft across the water surface.
Many internal combustion engines in outboard motors include four cylinders and operate on the four-stroke combustion cycle. The four-stroke combustion cycle is well known to those of skill in the art, and therefore will not be explained in detail here. Four-stroke engines comprise a crankcase in which the crankshaft is housed, a cylinder block extending generally horizontally from the crankcase, and a cylinder head extending generally horizontally from the cylinder block. The cylinder block defines four cylinder bores that are generally arranged vertically. The cylinder head defines four exhaust ports, with one exhaust port being associated with each cylinder. Each exhaust port expels exhaust gases into a runner, which provides a fluid communication path between its associated cylinder and an exhaust passage. The exhaust passage discharges the exhaust gases to the atmosphere.
Some four-cylinder four-stroke engines include first and second exhaust passages extending generally vertically through the cylinder block. An engine having a dual exhaust passage configuration is simpler and less expensive to construct than an engine with four exhaust passages. The exhaust ports of two of the cylinders communicate with the first exhaust passage, and the exhaust ports of the other two cylinders communicate with the second exhaust passage.
In order to reduce interference of the exhaust gas flow from the cylinders, the cylinders are paired according to the firing order. For example, the cylinders are paired such that the timing of the opening of the exhaust valves of the two cylinders communicating with the first exhaust passage are not sequential. Similarly, the timing of the opening of the exhaust valves of the two cylinders communicating with the second exhaust passage are not sequential. For example, if the first and fourth cylinders communicate with the first exhaust passage, and the second and third cylinders communicate with the second exhaust passage, then a preferred firing order would be 1-2-4-3.
Non-sequential exhaust timing eliminates interference between exhaust gases traveling through the same exhaust passage. If the exhaust times of the two exhaust ports communicating with the same exhaust passage were sequential, then the exhaust gas pulse from one cylinder could interfere with the next exhaust pulse from the other cylinder. Providing a temporal gap between exhaust pulses entering the same exhaust passage reduces the interaction between exhaust pulses discharged from the cylinders. Thus, non-sequential exhaust timing allows each cylinder to exhaust under equal pressure, leading to better engine performance.
In engines having non-sequential exhaust timing, the exhaust passages are arranged side by side in a lateral (port to starboard) direction. This arrangement increases an overall width of the engine. A sturdy plastic cowling typically encloses the engine. As the width of the engine increases, so must the width of the cowling. Ideally, however, the width of the cowling decreases gradually toward the top. This design provides the cowling with a more pleasing appearance and with more favorable aerodynamic properties. A wider engine compromises these two desirable properties of the cowling. Some outboard motors provide a tapered cowling even for very wide, non-tapering engines by creating a large lateral gap between the engine and the cowling at a lower end of the engine. However, this solution only expands the overall width of the cowling and wastes material for the cowling.
Because the exhaust passages are arranged side by side in a lateral direction, one of the exhaust passages is spaced farther from the cylinders. Therefore, the exhaust runners leading to the farther exhaust passage are longer than the exhaust runners leading to the nearer exhaust passage. The cylinder head defines exhaust runners. A cylinder head having exhaust runners of unequal lengths is more complex, and therefore more expensive to manufacture, than a cylinder head having exhaust runners of equal lengths.
Furthermore, the cylinders all have equal volumes. Therefore, gases passing through exhaust runners of different lengths experience different pressure losses. As explained above, cylinders exhausting under unequal pressure compromise engine performance. Prior attempts at eliminating the problems caused by laterally side-by-side exhaust passages and exhaust runners of unequal lengths have been unsuccessful.